The present invention relates to an optical privacy communication system in a two-way optical transmission system in which a plurality of stations are branch-connected to an optical fiber transmission line.
Optical fiber communication systems now in use utilize, as a repeating system, an optical-to-electric-to-optical conversion system which involves once converting an optical signal into an electric signal, amplifying and waveform-shaping the electric signal, and driving again a semiconductor laser with the amplified electric signal.
On the other hand, there has been proposed, as a new repeating system, an optical amplification system which directly amplifies an optical signal. The repeating system utilizing the optical amplification is advantageous over the conventional optical-to-electric-to-optical conversion repeating system in that repeating circuits used are simplified and that wavelength multiplex signals or frequency multiplex signals of a desired transmission rate can be amplified collectively by one optical amplifier. Further, since the amplifier gain is fixed regardless of the direction of incidence of light, optical signals in two ways can be amplified together by one optical amplifier.
The optical amplification is performed chiefly by a method using a semiconductor laser [Electronics Letters, Vol. 21, pp. 1076-1077, 1985] and an optical fiber Raman amplification method utilizing a stimulated Raman scattering phenomenon which occurs in an optical fiber [Electronics Letters, Vol. 23, pp. 196-197, 1987]. Up to now results of many studies have been reported on amplification characteristics and noise characteristics of these optical amplification methods. It is now predicted theoretically that the direct optical amplification will provide a maximum amplifier gain in the range of 30 to 40 dB and permit an optical transmission over 10,000 km with no influence of a background light noise which is caused by spontaneous scattered light, and the practicality of this system as a future optical repeating system is becoming higher (IEEE. J. Lightwave Tech., Vol. LT-4, pp. 1328-1333 and IEEE. J. Quantum Electron, Vol. QE-17, pp. 919-935). When this optical repeating system is put into practical use, an optical cable having a repeater will be considered as a mere optical transmission line just like a non-repeating optical cable, and the combined use of this system and an optical branching technique will make possible a multi-point communication in which a frequency (or wavelength) is assigned to each section between the points.
In this instance, the privacy of communication is an important problem to which special attention should be paid. Especially in international communications there is an eager demand for a privacy characteristic from the political point of view.